VOLUME 64 : 2016

VOLUME 64 : 2016

ACTA MANILANA publishes research and innovation in the different branches of the natural and applied sciences. It reports significant development in the discipline, and novel applications, unconfined by the traditional coverage of the disciplines.

Enantioselective chiral 2-pyridyl-2-imidazoline organocatalyzed Aldol reactions in brine

Page 33-39

Aileen Marice R. Baltazar, Simon Budde, Christian Faderl, Aldrick B. Verano, & Allan Patrick G. Macabeo

Graphical Abstract


Among the known organocatalyst manifolds, the organocatalytic utility of 2-pyridylimidazolines has been less explored. In this study, a collection of chiral 2-pyridyl-2-imidazolines (picolinylimidazolines) were screened for organocatalytic activity towards enantioselective direct Aldol reaction. The chiral imidazolines (1a-1d) were prepared through conventional iodine-promoted oxidative condensation and cyclization of 2-picolinaldehydes 2 (and/or 2,6-pyridine dicarbaldehyde) with chiral 1,2-diamine derivatives in excellent yields (>96% isolated yields). Organocatalytic parameters were optimized to determine the best Aldol reaction conditions that would induce enantioselectivity such as catalyst loading, temperature, reaction time and solvent. Thus, using a C2-symmetric cyclohexane-based 2-pyridyl-2-imidazoline organocatalyst 1c, the best conditions that gave excellent enantioinduction of up to 99:1 enantiomeric ratio (er) and yield were observed with brine as the solvent, a 1-h reaction period at room temperature and 10 mol% as the minimum catalyst load. The reaction conditions were also amenable to a variety of benzaldehyde substrates with electron-donating and electron-withdrawing substituents. Our study demonstrates for the first time the asymmetric construction of b-hydroxy carbonyl structures using chiral C2-symmetric 2-pyridyl-2-imidazolines as organocatalysts.

Keywords: Aldol reaction, organocatalysis, picolinylimidazoline, Beta-hydroxy carbonyl, asymmetric synthesis


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